Wireless communications are an important part of the modern information infrastructure. The last decade has been marked by the exponential spread of handsets, such as smartphones, tablets, as well as new network-dependent devices. The harsh requirements, in terms of high data-rate communication links, have driven the successive generations of standards with higher throughput, mobility-support and increased Quality-of-Service (QoS) and Quality-of-Experience (QoE). Nonetheless, all further progress must be done in a smooth and efficient manner without entailing in increased Operational Expenditure (OPEX)/Capital Expenditure (CAPEX) costs.
Increasingly, this implies that the radio resources must be efficiently exploited, at the same time that higher data-rate wireless access technologies must be developed. To meet the data-rate requirements in an efficient way, the first step involved the augment of the available bandwidths in the 3G systems. Then, in an attempt to achieve scalable wider bandwidths, without spectrum allocation constraints, the concept of CA was introduced in 4G systems, such as Long-Term Evolution (LTE)-Advanced.
By standardizing the contiguous and the non-contiguous CA capabilities, the combination of multiple frequency bands to conduct high-speed data transmission was enabled. Due to the commercial success of LTE-Advanced features, it is expected that they will continue to be evolved, as a part of 5G technologies. To accomplish the Radio Access Network (RAN) expectations in a compact and efficient way, there is a need for the development of flexible, agile and reconfigurable radio transceivers, with a native support for multiple bands and multiple standards. The integration of these features can provide an efficient answer to the establishment of multiple, concurrent and frequency-agile data links between all the RAN parties.
The concept of All-Digital Transmitter (ADT) has been targeted as a promising path towards the development of the next generation of Radio-Frequency (RF) transceivers. The promising potential to design compact and versatile wireless communication transceivers has attracted much and renewed attention. Some methods describe a fully digital datapath from Baseband (BB) up to the RF stage. This enables the design of low-complex and flexible transmitters. The underlying idea is the quantization of an m-bit digital signal into a 2-level representation, resulting in signals with constant envelope. After a digital upconversion to the desired carrier frequency, the pulsed representation can be amplified by highly-efficient and non-linear amplifiers, such as the Switched-Mode Power Amplifiers (SMPAs). After the amplification, a bandpass filter is required to reconstruct the signal before being radiated by the antenna. Their fully digital behavior inherently leads to agile, flexible, reconfigurable, multi-standard, and important for this work, multi-band RF front-ends with minimal external front-end.
Nevertheless, despite the apparent ideal and native support for the multiband capability, design challenges associated with the non-contiguous CA transmission have hampered the proposal of multi-band solutions. Multiband transmission can be achieved with integer multiples of the modulators sampling frequencies, or with reduced sampling rate topologies. Other methods employ bulky and inefficient power combiners to join different bands before transmission. Some of the difficulties in designing multi-band transmission arise from the placement of the Digital Up-Conversion (DUC) after the pulse encoding. Following this approach, as the encoded signals have a considerable amount of out-of-band noise distributed over the entire spectrum, the upconversion to the different bands typically leads to a degraded system performance. One methodology to achieve higher spans between bands is based on the utilization of replicas from different Nyquist Zoness (NZs). However, the inherent decrease in terms of Signal-to-Noise Ratio (SNR), associated with the need of maintaining an integer multiplicity in all the involved sampling rates/frequencies lead to a reduced performance. In addition, the positioning the DUC before the pulse encoding typically implies sampling rates of at least twice the carrier frequency. This imposes challenging requirements in both the Pulse Encoder and in the analog front-end. Accordingly, there is a need for a digital transmitter suitable for contiguous and non-contiguous multi-band transmission. It is also highly desired to look for solutions that overcome the design and implementation challenges of future directional communication system such as multi-band massive MIMO phased array system in 5G.